PROJECT SUMMARY Abstract: Dogs are a well-established animal model for human cancers and are affected by a spontaneously occurring, late onset, malignant histiocytic disorder that is histologically similar to human histiocytic disease. Canine histiocytic sarcoma (HS), like human HS, is extremely lethal, and while rare across dog breeds affects 20-25% of Bernese Mountain Dogs (BMDs) and Flat-coated Retrievers (FCRs). Genome-wide association studies indicate unique associations in each breed, including the well-known cancer gene CDKN2A/B in BMDs. The presence of distinct susceptibility loci is consistent with differences in disease presentation between the breeds. FCRs are twice as likely as BMDs to develop localized HS, with a tumor in the periarticular tissue surrounding a joint. BMDs are seven times more likely than FCRs to develop the second form of HS, disseminated or visceral, with tumors arising in multiple organs. These discrete associations in the unrelated breeds and differences in clinical presentation suggest that predisposition to HS arose twice in dogs, and there may be distinct mechanisms of tumorigenesis, increasing the utility of the model for human HS studies. The central hypothesis of the proposed work is that HS tumors possess commonly disrupted gene pathways related to tumorigenesis as well as HS subtype-specific mutations and differential expression profiles related to the underlying susceptibility mechanisms in BMDs and FCRs. The following specific aims will address this hypothesis: (1) characterization of the HS tumor transcriptome in BMDs and FCRs, (2) identification of somatic variation and mutational signatures in canine HS tumors, and (3) investigation of germline candidate susceptibility variants and comparison to results from tumor WGS and RNA-seq data. This comprehensive approach will provide novel therapeutic targets and susceptibility alleles of relevance to human disease. Fellowship Training Plan: The proposed work will take place at the National Human Genome Research Institute in the laboratory of Dr. Elaine Ostrander, a leader in the field of canine genetics and cancer research. The fellowship training plan includes courses in bioinformatics, python, Perl, and statistics; mentoring of a graduate student; presentations at human genetics, animal genomics, and cancer conferences; attendance at NIH lecture series, journal clubs, and weekly lab meetings; and career development courses in grant writing, workplace dynamics, and lab management. The specific aims will address the PI?s training goals, which are to learn conceptual and technical skills related to RNA-seq and WGS of tumors and to improve bioinformatics and statistical abilities to prepare the applicant for a career in canine genetics studying multigenic, complex disease.